1. Field of the Invention
The present invention relates to an image forming apparatus which forms a color image on a recording medium such as paper, particularly to an image forming apparatus which forms a color image by an electrophotographic method, and to an image forming method of the same.
2. Description of the Related Art
In an electrophotographic color image forming apparatus, a color image is formed by superimposing a plurality of single color images, for example, a yellow image, a magenta image, a cyan image, and a black image, on a recording medium such as paper.
Among color image forming apparatuses, there is one which forms a plurality of images by repeating a plurality of steps of forming a single color image, and one which forms a plurality of images in one step. In most cases, the latter type is employed as a high-speed image forming apparatus, since this type of image forming apparatus is capable of forming a color image and a black-and-white image at substantially the same speed.
The latter type of image forming apparatuses include: one which forms a color image by forming a single color image on each of a plurality of image forming bodies, transferring each of the single color images from the image forming bodies to a recording medium, and superimposing the single color images on the recording medium to form a color image thereon; and one which forms a color image by forming a single color image on each of a plurality of image forming bodies, temporarily transferring each of the single color images to an intermediate transfer body, superimposing the single color images to form a color image thereof, and transferring the color image from the intermediate transfer body to a recording medium.
Meanwhile, in an electrophotographic image forming process, a contact heating method such as heat roller fixing is generally used. In the contact heating method, offset, in which part of toner forming an image on a recording medium is transferred from the recording medium to a fixing member, easily occurs. Moreover, a recording medium easily sticks to a fixing member. Thus, in order to prevent these phenomena, a release agent has been contained in toner.
As described above, a release agent mainly acts to prevent offset and sticking of a recording medium to a fixing member. A release agent also acts in other ways. For example, in JP-Tokukaihei-8-101553A, it is proposed that the containing percentages of release agents are differed between chromatic toner and achromatic toner in order to form a black-and-white image without unnecessary gloss and a color image with appropriate gloss by using the same image forming apparatus.
In the abovementioned latter type of color image forming apparatus, in theory, it is assumed that the amount of heat required for fixing of toner is smaller in a black-and-white image forming, since the amount of the maximum toner load of a color image on a recording medium is larger than that of a black-and-white image, and thus a black-and-white image can be formed at a speed higher than that at which a color image is formed. However, when toner disclosed in JP-Tokukaihei-8-101553A is used, the amount of the release agent which seeps out from the black toner while a recording medium passes through a fixing unit is small, and it has therefore been difficult to allow the speed of forming a black-and-white image to be higher than that of forming a color image. Moreover, when the softening point or the containing percentage of a release agent in chromatic toner is substantially the same as that in achromatic toner, gloss of a black image in a black-and-white image is increased, and thus it has been impossible to finish the black-and-white image with suppressed, desirable gloss.
In addition, disadvantages of a release agent have been known. For example, when a release agent is exposed to a temperature equal to or higher than a glass transition point or to mechanical stress such as stirring, the release agent seeps out from toner particles to the surface thereof, thus causing the toner to easily harden. Moreover, the release agent may be moved to the surface of carriers, thus deteriorating the electrostatic property of the toner.
With regard to how a color image forming apparatus is actually used, in most cases, the apparatus is used for formation of black-and-white images rather than formation of color images.
In view of such situation concerning use of color image forming apparatuses, it is desired that image forming in a black-and-white mode be carried out at a speed higher than that in a color mode.
In order to realize the above, it is necessary that the fixing efficiency in a black-and-white is higher than that in a color mode.